The US military conducted two additional air strikes against Islamic State forces near the city of Arbil in northern Iraq this evening, the Pentagon said.

The air strikes, which follow the dropping of two 500-pound bombs on Islamic State positions earlier today, included a drone strike on a mortar position and an attack by four F/A-18 jets on an Islamic State convoy and mortar position, the Pentagon said in a statement.

Lawmakers in Washington have welcomed president Barack Obama’s decision to attack advancing Islamist militants in Iraq, but some questioned whether his administration has a long-term strategy to arrest Iraq’s disintegration.

The United States has a consulate and, since Iraq’s latest security crisis erupted in June, a joint military operations center staffed by 40 US servicemen in Arbil, capital of the semi-autonomous Iraqi Kurdistan region.

Members of the US Congress supported the air strikes, but Republicans demanded the president spell out a long-term strategy. Even Mr Obama’s closest Democratic allies made clear they wanted him to work with legislators, not circumvent them.

Today’s strikes were the first aggressive US military action in Iraq 2-1/2 years after Mr Obama withdrew the last American troops, fulfilling a promise he campaigned on to win office in 2008 and ending a bloody US war that began in 2003.

Mr Obama authorised air strikes late last night to avert “a potential act of genocide” of tens of thousands of members of the ancient Yazidi sect who have taken refuge on a desert mountaintop from Islamic State forces. The United States has also begun dropping relief supplies to the refugees.

The White House said today the military engagement would not involve ground forces. But reflecting Washington’s pressure on Iraqi politicians to form a government that includes Sunnis, Shias and Kurds, the White House said the authorisation for limited action could eventually include more military support to Iraqi security forces once the country forms a new “inclusive” government.

White House spokesman Josh Earnest said the initial US support would be military strikes to protect American personnel in Iraq and to address an urgent humanitarian situation.

But the United States also has a third goal “related to our belief and commitment to supporting integrated Iraqi security forces and Kurdish security forces as they unite the country to repel the threat” posed by Islamic State fighters, he said.

The Islamic State was defiant. A fighter told Reuters by telephone the US air strikes would have “no impact on us”.

“The planes attack positions they think are strategic, but this is not how we operate. We are trained for guerrilla street war,” he said. “God is with us and our promise is heaven. When we are promised heaven, do you think death will stop us?”

In Baghdad, where politicians have been paralysed by infighting while the state falls apart, the top Shia cleric all but demanded prime minister Nuri al-Maliki quit, a bold intervention that could bring the veteran ruler down.

Sheltering on mountain

Sunni fighters from the Islamic State, an al-Qaeda offshoot rejected as too extreme by Osama bin Laden’s successors, have swept through northern Iraq since June. Their advance has dramatically accelerated in the past week when they routed Kurdish troops near the Kurdish autonomous region in the north.

Attention has focused on the plight of Yazidis, Christians and other minority groups in northern Iraq, which has been one of the most diverse parts of the Middle East for centuries.

“The stakes for Iraq’s future can also not be clearer,” US secretary of state John Kerry said today. The Islamic State’s “campaign of terror against the innocent, including the Christian minority, and its grotesque targeted acts of violence show all the warning signs of genocide.”

The US Defense Department said planes dropped 72 bundles of supplies, including 8,000 ready-to-eat meals and thousands of gallons of drinking water, for threatened civilians near Sinjar, home of the Yazidis, ethnic Kurds who practice an ancient faith related to Zoroastrianism.

The Islamic State considers them to be “devil worshippers”. After fighters ordered them to leave, convert or die, most fled their towns and villages to camp out on Sinjar mountain, an arid peak where they believe Noah settled after the biblical flood.

“After we fled to the mountain, I returned one day to recover belongings and I saw the bodies of the elderly disabled men who had been shot dead by the Islamic State. They were too old to flee. I can’t forget that scene,” said Akram Edo, who escaped to Kurdish-held territory with seven children.

His brother Hameed Edo, still back on the mountain with five children, told Reuters by telephone water was running out and no aid had arrived for the civilians trapped in the wilderness.

Mahma Khalil, a Yazidi lawmaker in Baghdad, said: “We hear through the media there is American help, but there is nothing on the ground.... Please save us! SOS! save us!” he said. “Our people are in the desert. They are exposed to a genocide.”

Trample our dead bodies

In the Kurdish capital, suddenly near the front line for the first time after a decade of war, defiant residents said they were stockpiling weapons and prepared to defend the city.

“People with children took them to their families (outside Arbil), but the men have stayed,” said Abu Blind (44), working at a tea stall in Arbil bazaar. “They will have to trample over our dead bodies to reach Arbil.”

The Kurdish region has until now been the only part of Iraq to survive the past decade of civil war without a serious security threat. Its vaunted “peshmerga” fighters - those who confront death - also controlled wide stretches of territory outside the autonomous zone, which served as sanctuary for fleeing Christians and other minorities when Islamic State fighters arrived in the region last month.

But the past week saw the peshmerga crumble in the face of an advance by the fighters, who have heavy weapons they seized from Iraqi army troops that abandoned their posts in June. In addition, the fighters are flush with cash looted from banks.

Christians, many of them already refugees who had sought shelter in peshmerga-controlled areas, were suddenly forced to flee. Tens of thousands of Christians fled yesterday when the Islamic State overran their hometown, Qaraqosh.

A United Nations humanitarian spokesman said some 200,000 people fleeing the Islamists’ advance had reached the town of Dohuk on the Tigris River in Iraqi Kurdistan and nearby areas of Nineveh province. Tens of thousands had fled further north to the Turkish border, Turkish officials said.

‘Grave mistake’

While the relentless advance of Islamic State fighters has threatened to destroy Iraq as a state, bickering politicians in Baghdad have failed to agree on a new government since an inconclusive election in April.

Mr Maliki, a Shia Islamist whose foes accuse him of fuelling the Sunni revolt by running an authoritarian sectarian state, has refused to step aside for a less polarising figure, defying pressure from Washington and Tehran.

Grand Ayatollah Ali al-Sistani, a reclusive 84-year-old scholar whose word is law for millions of Shias in Iraq and beyond, has repeatedly pushed for politicians to break the deadlock and reunify the country. His weekly sermon today, read out by an aide, was his clearest call for Mr Maliki to go.

Though he did not mention Mr Maliki by name, he said those who cling to posts were making a “grave mistake”.

Reuters photographs yesterday showed the insurgents had raised their black flag over a checkpoint just 45 km from Arbil. US oil majors Exxon Mobil and Chevron began evacuating expatriate staff from Iraqi Kurdistan. Smaller oil companies also evacuated staff and cut back operations, and several saw their shares fall sharply on yesterday and today.

The Islamists’ lightning offensive and the threat of US military action sent shares and the dollar tumbling on world financial markets, as investors moved to safe haven assets such as gold and German government bonds.

Mr Obama, who brought US troops home from Iraq to fulfill a campaign pledge, insisted he would not commit ground forces and had no intention of letting the United States “get dragged into fighting another war in Iraq”.

Questions were quickly raised in Washington about whether selective US attacks on militant positions and humanitarian air drops would be enough to shift the balance on the battlefield against the Islamist forces.

“I completely support humanitarian aid as well as the use of air power,” Republican senator Lindsey Graham tweeted after <r Obama’s announcement. “However the actions announced tonight will not turn the tide of battle.”